The RSPCA has set itself in direct opposition to racing's ruling body by calling for a radical overhaul of the Grand National, despite a preliminary report from the British Horseracing Authority saying that the deaths of two horses in last month's race could not have been foreseen or prevented. The charity responded with its own report, calling for "seven key actions", including the removal of Becher's Brook, which it described as a "killer fence".

"Despite safety improvements, the Grand National is still too risky for the horses," said Gavin Grant, the chief executive of the RSPCA. "It's the unacceptable face of racing. We must not see horses dying year after year."

Grant said he had written to the BHA and to Aintree racecourse "raising major concerns" and calling for an "urgent meeting to discuss making the race safer". He said the race should be restricted to fewer than the present 40 runners, the fences should be "more forgiving" and drop landings should be eliminated.

The starting procedures should be changed, he added, while the siting of people intended to catch loose horses should be reviewed. Horses like Synchronised, killed in last month's race, who get loose before the start should be automatically withdrawn, he said.

Most controversially, he called for the removal of the iconic Becher's Brook, which the RSPCA believes was "instrumental in the deaths of both Synchronised and According To Pete".Another animal charity, World Horse Welfare, declined to comment until the BHA had issued its final conclusion, which may take months. Animal Aid, an animal rights organisation, said it would campaign for the National to be banned and for the BHA to be stripped of its regulatory function, on the grounds that it was "not fit for purpose".

But the RSPCA's reaction will cause most concern to the BHA, which has worked with the charity on welfare issues in the past and has pointed to that association as evidence of how seriously it takes the subject. A BHA spokesman declined to respond to Grant's statement, pending completion of the Authority's full report.

Earlier, the BHA had said it was "too early to speculate" whether any changes would be made to the race before next year's running. After an examination of TV footage, including some that has not been broadcast, its initial report found that Synchronised broke a leg while running loose about half a mile after he fell at Becher's Brook, the sixth fence on the National's first circuit. The report said the horse "appears to decelerate into the [11th] fence and does not jump it cleanly, dragging his hindlegs and hindquarters through the fence. It would appear he fractured his right hind tibia and fibula in the process".

The report discounted the possibility that a pre-race incident, when the horse unseated Tony McCoy and galloped loose, could have made any contribution to his eventual injury. He was examined by a vet after being caught and was judged fit to compete. His heart rate was "barely elevated above normal resting rate".

According To Pete broke his off-fore humerus when brought down at Becher's Brook on the second circuit. The report describes him jumping the fence "well" but having "nowhere to go", as On His Own had fallen in front of him.

However, the report notes that crowding at the fence may have made some contribution to the incident, On His Own having been "possibly left a little unsighted" as the leaders drifted in front of him from his right. This caused him "to get in close to the fence and consequently he landed steeply with reduced forward momentum, knuckling on landing".

The BHA found that all 40 riders had broken the rules before the start by lining up prematurely but decided there would be no disciplinary action, due to "mitigating circumstances". The Professional Jockeys' Association insisted that far fewer had broken the rules but promised to work with the BHA in considering what changes could be made to the start.